Use of dimiracetam in the treatment of chronic pain

ABSTRACT

The use of dimiracetam in the treatment of chronic pain is disclosed. At doses higher than those previously disclosed in relation with its cognition enhancing activity (i.e. amelioration of learning and memory), dimiracetam was able to completely revert hyperalgesia or allodynia associated with several animal models of chronic pain. Dimiracetam showed high activity in iatrogenic neuropathies associated with antiviral and chemotherapeutic drug treatments and in painful conditions caused by osteoarthritis. In addition, dimiracetam was devoid of toxicity even at doses 10-fold higher than the highest therapeutic dose. The possibility of treating such debilitating pathologies with a highly effective and essentially non-toxic compound is therefore disclosed.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a divisional of U.S. application Ser. No.12/450,944, filed Oct. 16, 2009, which is a National Phase ofPCT/EP2008/054553, filed Apr. 15, 2008, which claims priority fromItalian Application MI2007A000770, filed Apr. 16, 2007, all of which areincorporated herein by reference.

FIELD OF THE INVENTION

The present invention relates to the field of pharmacological treatmentof chronic pain.

BACKGROUND OF THE INVENTION

Differently from acute pain, which exerts an important physiologicalaction alerting the organism towards an incoming danger or damage,chronic pain is not involved in any protective action.

Chronic pain may be divided in two main categories: chronic inflammatorypain and neuropathic pain. The latter is due to a direct lesion on thenervous pathways by the noxa, which can be infectious, metabolic,vascular or other. In chronic inflammatory pain the lesioned tissuesrelease algogenic factors which in turn damage nerve terminals creatinga vicious mechanism which maintains and potentiates the perception ofpain (hyperalgesia) or transforms into pain other types of perception(allodynia).

Chronic pain, of both neuropathic and inflammatory origin, is animportant epidemiologic aspect of a high unmet medical need condition;in fact this is a therapeutic area presently characterized by modestlyeffective and poorly tolerated treatments.

An increasing number of patients suffer from iatrogenic neuropathicpain, induced by anti-tumor therapies used in modern oncology. Inparticular taxol derived drugs, cisplatin and vincristine are among thedrugs which more often induce painful neuropathies. Currently noeffective and/or well tolerated treatments exist for this kind of pain.In fact classical antiepileptic or antidepressive agents successfullyused in other forms of neuropathic pain, such as lamotrigine (Renno S.I. 2006, J. Clin. Oncol. ASCO Annual Meeting Proceeding Part I vol. 24,No 18S:8530), gabapentin (Wong G. Y. 2005, J. Clin. Oncol. ASCO AnnualMeeting Proceeding Part I vol. 23, No 16S:8001) or nortriptyline(Hammack J. E. 2002, Pain 98:195-203) are absolutely unsatisfactory onthe basis of their therapeutic index.

Nucleoside analogue reverse transcriptase inhibitors (ddC, d4T, AZT) arecommonly used as antiviral drugs in the treatment of AIDS. These drugsoften cause the insurgence of peripheral neuropathies with differentdegrees of severity after prolonged treatment. As in the case ofchemotherapeutic agents, these symptoms may be so strong to induceshortening or suspension of these life-saving therapies. The patterns ofthese neuropathies are clearly different from those induced by theprogression of AIDS; they are in fact characterized by the sudden onsetof very intense burning discomfort in both hands and feet at about thetenth week of treatment. HIV-induced neuropathies, on the contrary, havea very slow progression (Dubinsky R. M. 1989, Muscle Nerve 12:856-860).As for chemotherapy-induced neuropathies, it is difficult to treat thiskind of pain.

The tricyclic antidepressant amitryptiline and the sodium channelblocker mexiletine, effective on various forms of painful peripheralneuropathies, did not show any significant effect on this kind ofneuropathic pain (Kieburtz K. 1998 Neurology 51:1682-1688). Gabapentinshowed some efficacy, although patients with severe syndromes rarelyreach satisfactory results and the additional administration ofnarcotics is required (McArthur J. C. 2001, The Hopkins HIV report.http://www.hopkins-aids.edu/publications/report/may01_(—)2.html).

Other forms of neuropathic pain may be caused by viral infections.Postherpetic neuralgia, for instance, is caused by the reactivation,long after the infection, of the varicella-zoster virus. This kind ofneuropathy is characterized by the development of strong mechanicalallodynia, frequent loss of sensitivity towards thermal stimuli andspontaneous intermitting pain. Pain intensity compromises the quality oflife of patients suffering from this condition.

Of high epidemiological relevance is the pain referred to ascephalalgia. This is localized to the head, face and neck. When cephaleaoccurs in a paroxystic way, with recurrent episodes lasting from hoursto days and is associated to general sickness, it is called migraine.Several forms of migraine are recognized such as common, classical,hemiplegic, vertebro-basilar, etc.

The current treatment for migraine entails the use of different kinds ofanalgesic agents, from non-steroidal anti-inflammatory drugs (NSAIDs) toopioids, antihistaminic drugs and ergotamine derivatives. In the lastdecade triptan 5HT2 antagonists have been used; they are often able toblock an attack at its insurgence, if promptly administered. All thesetreatments show serious limits in terms of both efficacy andtolerability. In the most severe cases, in which painful attacks recurmany times a week, a pre-emptive therapy with antiepileptic, betablocker and antidepressant drugs is performed. The maximum result whichcan be achieved with these pre-emptive therapies is 50% reduction in thefrequency and intensity of the painful attacks, but not their definitiveremission.

Inflammatory pain is another form of chronic pain. It is caused by therelease of mediators which either directly activate the nociceptorslocalized on primary afferents, or lower their activation threshold,thus increasing their sensitivity to either painful or non-painfulstimuli of different nature. Excited primary afferents may in turnrelease neurotransmitters which can stimulate immune cells recruited bythe inflammatory process causing the release of additional inflammatorymediators.

This phenomenon, defined ‘neurogenic inflammation’, leads to anautoamplification of the symptomatology of the patient. Osteoarthritisis a particularly severe and painful form of this kind of pathology.Osteoarthritis is a form of degenerative arthritis causing the breakdownand eventual loss of the cartilage of one or more joints. The mostcommon symptom related to this pathology is pain in the affected jointafter repetitive use or after prolonged periods of inactivity (night andrest pain). Even if a certain correlation between pain and the extensionof the damage at the joint has been demonstrated, the precise etiologyof this kind of pain is still obscure; in fact, patients with relativelysmall damages at the joints suffer from very intense pain and viceversa; this finding suggests that it is not a merely inflammatory pain,but that a neuropathic component is present as well. Recommendedtreatments include NSAIDs, steroids and opioids, but the use of thesedrugs is associated with the insurgence of severe side-effects; inaddition, they do not show full efficacy in many instances (Altman R. D.2000 Arthritis Rheum. 43:1905-1915).

The fibromyalgia syndrome is the most frequent cause of chronic,widespread pain, associated with auxiliary symptoms, such as sleepdisturbances and chronic fatigue (Rao S. G. 2007, Psychopharmacol. Bull.40:24-67). Nearly 2% of the general population in the United Statessuffers from fibromyalgia, with females of middle age being at increasedrisk. Patients with fibromyalgia display quantitative abnormalities inpain perception under experimental conditions, in the form of bothallodynia and hyperalgesia: these data are suggestive of a state ofsensitized pain perception.

Recently, pregabalin and duloxetine showed some efficacy in clinicaltrials for the treatment of the muscle pain in fibromyalgia (Crofford L.J. 2005, Arthritis Rheum. 52:1264-1273; Maizels M. 2005, Am. Fam.Physician 71:483-490). Nonetheless, at present, the medical treatmentfor pain relief in fibromyalgia is unsatisfactory (Offenbaecher M. 2005,CNS Spectr. 10:285-297) and fibromyalgia represents a high unmet medicalneed.

Dimiracetam (2,5-dioxohexahydro-1H-pyrrolo[1,2-a]imidazole) is abicyclic pyrrolidinonic derivative of formula (I)

Patent application EP-A-335483 claims its pharmaceutical use as anootropic agent, i.e. able to improve learning and memory in humans andanimals. Dose-response data show that the nootropic activity ofdimiracetam tends to lower for oral doses greater than 10 mg/kg (J. Med.Chem., 1993, 36:4214-4220). Patent application WO-A-93/09120 claims aprocess for the preparation of dimiracetam and of its enantiomers.

WO-A-2004/085438 claims a set of derivatives of2,5-dioxohexahydro-1H-pyrrolo[1,2-a]imidazole; a typical feature ofthese compounds is the presence, in position 3 of the imidazole ring, ofan aromatic carbocyclic or heterocyclic ring; these compounds,notwithstanding their utility in the treatment of painful conditions,show a therapeutic index which is not fully satisfactory.

In view of the above mentioned background the need is felt for new drugsendowed with high antihyperalgesic and antiallodynic activity towardschronic pain, and characterized by a high therapeutic index. The need isalso felt for the treatment of specific forms of neuropathic pain whichare not well treated with traditional antihyperalgesic agents.

SUMMARY OF THE INVENTION

The present inventors have studied the behaviour of dimiracetam atdifferent doses with respect to those previously described for thiscompound, considering also possible variations of toxicity associated tothe new doses. During these studies a new pharmacological window hasbeen found, within which dimiracetam exerts a strong regression effecton chronic painful phenomena of neuropathic or inflammatory origin,without showing any toxic effect. The possibility to treat thesedebilitating pathologies with an effective and essentially atoxiccompound is therefore disclosed.

BRIEF DESCRIPTION OF FIGURES

FIG. 1: Oxaliplatin-induced neuropathy

*p<0.01 vs oxaliplatin/vehicle treated group. Each value represents themean±S.E.M. of 8-11 rats. Compounds were administered starting threedays before oxaliplatin treatment.

FIG. 2: ddC-induced neuropathy

*p<0.01, ^p<0.05 vs ddC/vehicle group. Each value represents themean±S.E.M. of mechanical threshold expressed as grams, with a total of10 rats per group.

FIG. 3: ddC-induced neuropathy

*p<0.01 vs ddC/vehicle group. Each value (with the exception of thecontrol group) represents the mean±S.E.M. of 18 rats in two experiments.

FIG. 4: MIA-induced osteoarthritic pain in rats

*p<0.01 vs MIA/vehicle group. Each value represents the mean±S.E.M. of18 rats in two experiments.

FIG. 5: Motor coordination in rats (rotarod)

Each value represents the mean±S.E.M. of the number of falls in 30 sec.of groups of 8 rats.

FIG. 6: Motor coordination in rats (rotarod)

Each value represents the mean±S.E.M. of the number of falls in 30 sec.of groups of 8 rats. *p<0.01 vs vehicle-treated animals.

FIG. 7. Motor activity in mice (hole board)

*p<0.01 vs vehicle treated group. Each value represents the mean±S.E.M.of 18 mice. The test was performed 30 min after the oral administrationof drugs.

DETAILED DESCRIPTION OF THE INVENTION

A first object of the present invention is the use of dimiracetam, or apharmaceutically acceptable solvate thereof, in the manufacture of amedicament useful for treating and/or preventing chronic pain. Theinvention is also directed to dimiracetam, or a pharmaceuticallyacceptable solvate thereof, for use in the treatment and/or preventionof chronic pain.

A further object of the present invention is a method for treatingand/or preventing chronic pain, consisting in the administration of apharmaceutically effective dose of dimiracetam to a patient in needthereof.

Dimiracetam is a chiral compound. For the scope of the presentinvention, the term “dimiracetam” identifies the isolated (R) or (S)enantiomers of dimiracetam, or mixtures thereof in which the twoenantiomers are present in equal or different amounts. It is thereforeintended that the use, method and pharmaceutical compositions which arethe object of the present invention are extended to those mixtures orthe single enantiomers of dimiracetam.

According to the present invention, dimiracetam may be administered assuch or in association with any other active principle useful for thetreatment or prevention of chronic pain or diseases causing it.

It is also part of the invention the administration of dimiracetam inassociation with active principles which present as side effect theinsurgence of chronic pain, in particular antitumor and antiviral drugs;non-limiting examples of such drugs are taxol, vincristine, cisplatin,oxaliplatin, nucleoside reverse transcriptase inhibitor antivirals (ddC,d4T, AZT), many of which are gold standard antiviral drugs in HIVinfection therapy.

By means of the claimed use and method it is possible to treateffectively and with high safety all kinds of chronic pain, eitherneuropathic or inflammatory in origin. Preferred examples of chronicpain treated according to the present invention are the following:

-   -   1. pain induced by chemotherapeutic agents or other antiblastic        therapy (e.g. radiotherapy); among the chemotherapeutic agents        responsible for neuropathies, taxol, vincristine, cisplatin,        oxaliplatin are mentioned;    -   2. pain induced by antiviral agents such as nucleoside reverse        transcriptase inhibitors (ddC, d4T, AZT);    -   3. complex regional pain syndrome, phantom limb, thalamic        syndromes, spinal syndromes;    -   4. pain induced by osteoarthritis, rheumatoid arthritis,        autoimmune osteoarthrosis forms;    -   5. pain induced by cephalea (cephalea in general and hemicranic        forms; cephalea due to vascular, infective, autoimmune,        dysmetabolic and tumoral causes, cephalea from endocranial        hypertension, cephalea from pseudotumor cerebri, classic        hemicrania with and without aura, hemiplegic hemicrania and with        other motor complications, childhood and juvenile hemicrania,        Bickerstaff's syndrome, etc.).    -   6. pain induced by fibromyalgia

Of outstanding efficacy, and therefore preferred in the scope of theinvention, is the treatment of pain induced by antiviral agents,osteoarthritis, rheumatoid arthritis and autoimmune osteoarthritis.

In the scope of the invention, in the present treatment theantihyperalgesic effect of dimiracetam is exerted in a range of oraldosages between 10 and 300 mg/kg, preferably between 100 and 300 mg/kg.The antihyperalgesic effect may be achieved also by routes ofadministration different from the oral route, i.e. intramuscular orintravenous: in these cases dimiracetam is administered in amounts whichallow to obtain haematic levels comparable to those obtained after oraladministration of 10-300 mg/kg. Reference values useful forintramuscular administrations range from about 5 to about 150 mg/kg;reference values useful for intravenous administrations range from about2 to about 60 mg/kg.

The invention encompasses therefore pharmaceutical compositions ofdimiracetam useful for the above mentioned treatments. Thesecompositions contain an amount of this active principle which is greaterthan that previously proposed for the nootropic activity.

The amounts of the active principle, expressed in mg/kg, are those citedabove. These compositions have a dosage unit useful to administer theabove mentioned dosages. Typically they contain from 500 to 15000 mg incase of oral compositions; from 250 to 7500 mg in case of intramuscularcompositions; from 100 to 3000 mg in case of intravenous compositions.

Dimiracetam may be pharmaceutically formulated according to knownmethodologies. The various pharmaceutical compositions may be selectedaccording to the needs of the treatment.

Such compositions can be prepared by mixing and can be suitably adaptedfor oral or parenteral administration, and as such, can be administeredin the form of tablets, capsules, oral preparations, powders, granules,pellets, liquid solutions for injection or infusion, suspensions orsuppositories.

Tablets and capsules for oral administration are usually supplied indosage units and may contain conventional excipients such as binders,fillers, diluents, tabletting agents, lubricants, detergents,disintegrants, colorants, flavors and wetting agents. Tablets may becoated in accordance to methods well known in the art.

Suitable fillers include for example cellulose, mannitol, lactose andsimilar agents. Suitable disintegrants include starch,polyvinylpyrrolidone and starch derivatives such as sodium starchglycolate. Suitable lubricants include, for example, magnesium stearate.Suitable wetting agents include for example sodium lauryl sulfate.

These solid oral compositions can be prepared with conventional mixing,filling or tabletting methods. The mixing operations can be repeated todisperse the active agent in compositions containing large quantities offillers. These operations are conventional.

The oral liquid compositions can be provided in the form of, forexample, aqueous or oily suspensions, solutions, emulsions, syrups orelixirs or in the form of a dry product to be reconstituted with wateror with a suitable liquid carrier at the time of use. The liquidcompositions can contain conventional additives such as suspendingagents, for example sorbitol, syrup, methylcellulose, gelatin,hydroxyethylcellulose, carboxymethylcellulose, aluminium stearate gel orhydrogenated edible fats, emulsifying agents, for example lecithin,sorbitan monooleate, or acacia; non aqueous carriers (which can includeedible oil) for example almond oil, fractionated coconut oil, oilyesters such as glycerin esters, propylene glycol or ethyl alcohol;preservatives, for example methyl or propyl p-hydroxybenzoate or sorbicacid and if desired, conventional flavours or colorants. Oralformulations also include conventional sustained release formulations,such as tablets or granules with enteric coating.

For parenteral administration, fluid dosage units can be preparedcontaining the active compounds and a sterile carrier. The activecompounds, depending on the carrier and concentration, can be suspendedor dissolved. The parenteral solutions are normally prepared bydissolving the compound in a carrier and sterilizing by filtration,before filling suitable vials or ampoules and sealing. Adjuvants such aslocal anaesthetics, preservatives and buffering agents can beadvantageously dissolved in the carrier. In order to increase stability,the composition can be frozen after filling the vial and the waterremoved under vacuum. The parenteral suspensions are preparedessentially in the same way, with the difference that the activecompounds can be suspended rather than dissolved in the carrier, and canbe sterilized by exposure to ethylene oxide prior to being suspended inthe sterile carrier. A surfactant or humectant can be advantageouslyincluded to facilitate uniform distribution of the compound of theinvention.

A further method of administration for the compound of the inventionrefers to a topic treatment. Topic formulations may contain for exampleointments, creams, lotions, gels, solutions, pastes and/or may containliposomes, micelles and/or microspheres.

A further method of administration for the compounds of the invention istransdermal delivery. Typical transdermal formulations includeconventional aqueous and non-aqueous vectors, such as creams, oil,lotions or pastes or may be in the form of membranes or medicatedpatches.

As is the common practice, the compositions are normally accompanied bywritten or printed instructions, for use in the treatment concerned.

Examples of the present invention are provided in what follows, purelyfor illustrative and non-limiting purposes.

EXPERIMENTAL PART 1. Methods

1.1 Chemotherapy-Induced Peripheral Neuropathy (CIPN)

Peripheral neuropathy is induced by repeated administration ofvincristine, taxol or oxaliplatin to adult male Sprague-Dawley rats(150-200 g, supplier Harlan).

The following protocols were used respectively:

-   -   Vincristine: the drug was injected by intravenous route at the        dose of 150 μg/kg. The treatment was performed every 2 days, for        5 times, until a cumulative dose of 750 μg/kg was reached. Paw        pressure test was performed 4 days after the last injection        (Marchand F. et al. 2003, Brain Res. 980:117-120).    -   Taxol: taxol neuropathy was induced by intraperitoneal        administration of 0.5 mg/kg once a day, on days 1, 3, 5 and 8.        Cumulative taxol dose was 2 mg/kg. The pharmacological test was        performed 14-18 days after the last taxol injection        (Polomano R. C. et al. 2001, Pain 94:293-304).    -   Oxaliplatin: 2.4 mg/kg were injected by intraperitoneal route        for 5 consecutive days followed by 2 days suspension (one        cycle). A total of 3 cycles was performed, reaching a cumulative        dose of 36 mg/kg (Cavaletti G. 2001, Eur. J. Cancer        37:2457-2463). The test was performed 48 h after the last        oxaliplatin injection.        1.2 Antiviral-Induced Neuropathy

Adult male Sprague Dawley rats (150-200 g, supplier Harlan) were treatedby intravenous route with a single administration of 25 mg/kg ofnucleoside reverse transcriptase inhibitors ddC (2′,3′-dideoxycytidine)or d4T (2′,3′-didehydro-3′-deoxythymidine). Administration of theseanti-HIV drugs induced a marked allodynic response to a mechanicalstimulus (Joseph E. K. 2004, Pain 107:147-158). The maximum reduction ofthe paw pressure threshold is developed between day 5 and day 10 afterinjection. The test was performed on day 10.

1.3 Cephalea

Experimental models in rats demonstrated that meninges and cerebralblood vessels are pain-sensitive structures and are heavily innervatedby the trigeminal nerve. Activation of trigeminal fibers causes aneurogenic inflammatory response of meningeal tissues, that has beenproposed as an essential mechanism for migraine pain and otherheadaches. (Bolay H. 2002, Nature Medicine 8:136-142). On these basis,animal models of blood vessel neuro-inflammation following electricaltrigeminal stimulation were commonly utilized to discover potentialeffective drugs. Adult male Sprague-Dawley rats (150-200 g weight,Harlan) were anaesthetized with pentobarbital Sodium® (60 mg/kg i.p.),and placed in a stereotaxic frame. An ipsilateral electrode was theninserted and trigeminal nucleus was stimulated to induce a meningealneuroinflammation, which can be detected by the amount of extravasatedBlue Evans dye or radiolabelled bovine serum albumine.

1.4 Arthritic Pain in Rats

Joint inflammation was induced by intra-articular injection of 0.1 ml ofFreund's complete adjuvant (CFA) in anaesthetized rats (male adultSprague Dawley rats, 150-200 g, supplier Harlan). Mechanicalhyperalgesia was evaluated using the paw pressure test 14 days after CFAadministration (Shan S 2006, Pain 129:64-75).

1.5 Osteoarthritis-Related Pain in Rats

Osteoarthritis was induced by a single administration of 2 mg (in avolume of 25 μl) of sodium 2-iodoacetate into the left knee joint ofanaesthetized rats (male adult Sprague Dawley rats, 150-200 g, supplierHarlan) (Fernihough J. 2004, Pain 112:83-93). This treatment induces theprogressive degeneration of the joint and the development ofhyperalgesia, mimicking at the histological and behavioral levels whatobserved in humans. Pharmacological test was performed 7 days aftertreatment.

1.6a Evaluation of Mechanical Hyperalgesia: Paw Pressure Test

Mechanical hyperalgesia in rats (male adult Sprague Dawley rats, 150-200g, supplier Harlan) was determined using the paw pressure test. Thenociceptive threshold was determined with an analgesimeter (Ugo Basile,Italy), exerting a force that increases at constant rate (32 g/s)according to the method described by Leighton G. E. 1988, Br. J.Pharmacol. 93:553-560. The stimulus causing paw withdrawal was evaluatedbefore and at different times after treatment. Results represent themean of mechanical thresholds expressed as grams. To avoid any possibledamage to the animal paw the maximum applied force was fixed at 240 g.

1.6b Evaluation of Mechanical Allodynia: Von Frey Test

Rats (male adult Sprague Dawley rats, 150-200 g, supplier Harlan) wereplaced in a chamber with a mesh metal floor covered by a plastic domethat enabled the animals to walk freely, but not to jump. The mechanicalstimulus was delivered in the mid-plantar skin of left hind paw using anelectronic von Frey apparatus. The cut-off was fixed at 50 g, while theincreasing force rate (ramp duration) was settled at 20 sec.

1.7 Irwin Test in Rats

To verify if the administration of the compound may induce centrallymediated side effects, adult male Sprague Dawley rats (150-200 g,supplier Harlan) were treated with dimiracetam by subcutaneous and oralroutes and monitored according to the “Irwin test” protocol (Irwin 1968,Psychopharmacologia 13:222-257), a systematic and quantitative procedurefor assessing the behavioral and physiological modifications induced inanimals by the drug treatment.

Rats were constantly monitored for 30 min after administration.Monitoring was iterated every morning at 9 a.m. for 4 days afteradministration.

1.8 Motor Coordination in Rats

The rotarod test allows the evaluation of the effects of a compound onmotor coordination. Adult male Sprague Dawley rats (200-220 g, supplierHarlan, Milan) were placed on a plastic rod 6 cm in diameter and 35 cmin length, rotating at constant speed (16 rpm) at a height of 25 cm. Therod is divided in 4 equal sections, thus up to 4 animals may be testedsimultaneously. The animals were required to walk against the motion ofthe rotating drum over 30 seconds. The time taken to fall off therotarod was recorded as number of falls in 30 seconds, following themethod of Vaught et al. 1985, Neuropharmacology 24:211-216. In eachexperiment motor coordination is measured before (pre-test) and afteradministration of the tested compound. Rats scoring less than 3 and morethan 6 falls in the pretest are rejected.

1.9 Rotarod/Ataxia Test in Rats

The test was performed according to the method described by Veneroni etal 2003, Pain 102:17-25. Neurological deficits were evaluated by theinability of the rats to remain on the rotating rod (10 rpm) for thetest period. The toxic dose was calculated as the dose causing 25%(TD₂₅) or 50% (TD₅₀) of the fallen rats (only for gabapentin, the toxicdose was TD₁₆=16% of fallen rats).

1.10 Hole Board in Mice

The hole board test allows to study the behavior of rodents whenconfronted with a new environment (Boissier J R 1964, Therapie19:571-583). The test enables to evaluate the initial exploratoryactivity of the animal and its possible variations induced by drugadministration.

The hole board test uses a 40 cm square plane with 16 flush-mountedcylindrical holes (diameter 3 cm) distributed 4 by 4 in an equidistant,grid-like manner. Mice (male Swiss Webster mice weighing 25-30 g,supplier Morini) are placed one by one in the center of the board andallowed to move freely, each for a period of 5 min. Two photoelectricbeams, crossing the plane from mid-point to mid-point of opposite sides,and thus dividing the plane into four equal quadrants, automaticallyrecord the movements of the animals on the plane surface. Miniaturephotoelectric cells in each of the 16 holes record the exploration ofthe holes (head plunging activity) by the mice.

2. Results (Antihyperalgesic Activity)

2.1 Oxaliplatin-Induced Neuropathy in Rats

The effect of dimiracetam was evaluated in the oxaliplatin-inducedneuropathy model after repeated administration with the paw pressuretest. Results are reported in FIG. 1. Dimiracetam was administered atdoses of 100 and 300 mg/kg p.o. once a day, starting three days beforeoxaliplatin treatment and during the treatment itself. At the dose of300 mg/kg, dimiracetam significantly reduced mechanical hyperalgesia.The effect was statistically significant between 30 min and 4 h afteradministration.

2.2 Antiviral-Induced Neuropathy

Test results (von Frey test) are reported in FIG. 2. At the dose of 100mg/kg, 15-30 min after administration, dimiracetam fully revertedddC-induced allodynia, the mechanical threshold being at the same levelin treated and control animals. The effect was still statisticallysignificant 45 min after treatment.

Dimiracetam is a racemic compound; the two corresponding enantiomerswere synthesized and separately tested in the ddC-induced neuropathymodel. The two compounds were administered orally at doses of 150 and300 mg/kg and their antihyperalgesic activity was evaluated with the pawpressure test. Results are reported in FIG. 3. (R)-dimiracetam induced asignificant reduction of the pain mechanical threshold at 300 mg/kg,15-45 min after administration. The (S) enantiomer induced a significanteffect at 300 mg/kg, 15 min after administration. These data demonstratethe efficacy also of the single enantiomers of dimiracetam.

2.3 Osteoarthritic Pain in Rats

The antihyperalgesic potential of dimiracetam was evaluated (pawpressure test) in the osteoarthritic pain model induced by the intrajoint injection of sodium monoiodoacetate (MIA). Test results arereported in FIG. 4. Both dimiracetam and its (R) enantiomer at the doseof 150 mg/kg, 15-30 min after administration, showed a statisticallysignificant effect in reverting MIA-induced hyperalgesia. At the dose of300 mg/kg dimiracetam fully reverted MIA-induced hyperalgesia, themechanical threshold being at the same level in treated and controlanimals between 15 and 45 min after administration; the effect was stillstatistically significant 60 min after administration. The effect of the(R) enantiomer was still statistically significant 45 min aftertreatment.

3. Results (Tolerability)

In order to verify if dimiracetam may induce unwanted side effects, thecompound was tested in the rotarod model (motor coordination and ataxia)in rats and in the hole board model (spontaneous and exploratoryactivity) in mice.

3.1 Rotarod Test in Rats

In acute toxicity experiments, dimiracetam, administered at 3000 mg/kgp.o. (20-fold the dose used in the previous pharmacological activitytests) does not alter rats motor coordination in the rotarod test, asshown in FIG. 5.

Differently, as shown in FIG. 6, reference compoundI-(3-cyanophenyl)-tetrahydropyrrolo[1,2-a]imidazole-2,5-dione(representative of compounds of formula (I) of WO2004/085438, seeexample 13) significantly altered animals motor coordination, increasingthe number of falls starting from the dose of 300 mg/kg; these data showa lower tolerability level for the said reference compounds.

3.2 Rotarod/Ataxia Test in Rats

The TD₂₅ of dimiracetam was 6000 mg/kg p.o., thus demonstrating a veryhigh safety and tolerability of the compound.

Among the reference standards, tramadol exhibited the highest toxicity,with a TD₅₀ of 253 mg/kg p.o., while pregabalin and levetiracetam showedTD₅₀s of 536 and 2000 mg/kg p.o. respectively. Gabapentin showed a TD₁₆of 1000 mg/kg p.o.

3.3 Irwin Test in Rats

Dimiracetam administered at the dose of 1000 mg/kg by subcutaneous routeand at the dose of 3000 mg/kg p.o. did not show any effects on all thebehavioral parameters observed.

3.4 Hole Board Test in Mice

In the hole board test, dimiracetam, administered at 3000 mg/kg p.o.does not significantly reduce either spontaneous activity (number ofmovements of each animal on the plane) or curiosity (number of headplungings), as shown in FIG. 7.

On the contrary, gabapentin administered at 1000 mg/kg causes astatistically significant reduction of both the evaluated parameters.

3.5 Preliminary Toxicity in Rats: Single Dose by Oral and IntravenousRoute

Oral or intravenous administration of a single dose of 3000 mg/kg ofdimiracetam to Sprague Dawley rats is substantially well tolerated. Nosigns of toxicity were observed during the experiment. Behavioralobservation, blood and urine analyses did not show any dose-relatedvariation of the measured clinical parameters.

3.6 Repeated Toxicity in Rats: 4 and 13 Weeks P.O.

Oral repeated administration of dimiracetam to Sprague Dawley rats, for4 weeks and up to a maximal dose of 2500 mg/kg/day did not produce anychanges in terms of mortality, symptomatology or changes of the normalbehavior.

Oral repeated administration of dimiracetam to Sprague Dawley rats for13 weeks and up to a maximal dose of 2500 mg/kg/day was well tolerated.No mortality or relevant clinical signs, as well as changes in terms ofbody weight, water and food consumption or in body temperature were seenat all dose levels. Hematology, clinical chemistry, coagulationparameters and urinalysis did not reveal drug related variation of thedifferent parameters evaluated at all tested doses. No macro- ormicroscopic lesions or abnormalities correlated with the administrationof dimiracetam were noticed.

3.7 Repeated Toxicity in Cynomolgus Monkeys: 4 and 13 Weeks P.O.

Oral repeated administration of dimiracetam in Cynomolgus monkeys for 4weeks and up to a maximal dose of 2000 mg/kg/day, was well tolerated bythe animals. A slight reduction in food consumption and body weight wasobserved in some animals treated with the maximal dose of 2000 mg/kg.

Oral repeated administration of dimiracetam in Cynomolgus monkeys for 13weeks and up to a maximal dose of 2000 mg/kg/day was well tolerated bythe animals. No mortality or relevant clinical signs, as well as changesin terms of body weight, water and food consumption or in bodytemperature were seen at all dose levels. Hematology, clinicalchemistry, coagulation parameters and urinalysis did not reveal drugrelated variation of the different parameters evaluated at all testeddoses. No macro- or microscopic lesions or abnormalities correlated withthe administration of dimiracetam were noticed.

Taken together, these data show the insurgence of a strongantihyperalgesic activity for dimiracetam within the dosage rangestypical of the present invention. The high potency of action isconfirmed by the fact that this compound showed remarkably higherefficacy than gabapentin, considered up to now the gold standard inchronic pain treatment therapy. Activity was found versus chronic painof different origins (i.e. chemotherapy-induced pain, antiviral-inducedpain, osteoarthritic pain, cephalea etc.) demonstrating the broadspectrum of applicability of the treatment proposed herein. In addition,data shown in said animal models highlight a special efficacy ofdimiracetam versus chronic pain associated with antiviral treatment andosteoarthritic pain and related pathologies. In addition, at dosestypical for the present invention, dimiracetam proved to be much moretolerable than gabapentin or pyrroloimidazole derivatives of prior art.

1. A method for the treatment of pain induced by osteoarthritis,rheumatoid arthritis, wherein a pharmaceutically effective amount ofdimiracetam, is administered to a subject in need thereof.
 2. The methodof claim 1 wherein the administration is performed either: orally in anamount of dimiracetam ranging from 10 to 300 mg/kg; or intramuscularlyin an amount of dimiracetam ranging from 5 to 150 mg/kg; orintravenously in an amount of dimiracetam ranging from 2 to 60 mg/kg. 3.The method of claim 2, wherein the administration is performed via adosage unit either: oral, wherein dimiracetam is present in amountsranging from 500 to 15000 mg; intramuscular, wherein dimiracetam ispresent in amounts ranging from 250 to 7500 mg; intravenous, whereindimiracetam is present in amounts ranging from 100 to 3000 mg.
 4. Themethod of claim 1 for the treatment of pain induced by osteoarthritis.5. The method of claim 1 for the treatment of pain induced by rheumatoidarthritis.
 6. The method of claim 1 wherein the administration isperformed orally in an amount of dimiracetam ranging from 10 to 300mg/kg.
 7. The method of claim 1 wherein the administration is performedintramuscularly in an amount of dimiracetam ranging from 5 to 150 mg/kg.8. The method of claim 1 wherein the administration is performedintravenously in an amount of dimiracetam ranging from 2 to 60 mg/kg. 9.The method of claim 2, wherein the administration is performed via anoral dosage unit wherein dimiracetam is present in amounts ranging from500 to 15000 mg.
 10. The method of claim 2, wherein the administrationis performed via an intramuscular dosage unit wherein dimiracetam ispresent in amounts ranging from 250 to 7500 mg.
 11. The method of claim2, wherein the administration is performed via an intravenous dosageunit wherein dimiracetam is present in amounts ranging from 100 to 3000mg.